A typical requirement in military systems is that unencrypted, classified (RED) data be electromagnetically isolated from encrypted, unclassified (BLACK) data in order to ensure the security of the classified information. Additional stringent measures, such as TEMPEST measures, are often required to control comprising emanations. Military specifications, such as MIL-HDBK-232A RED/BLACK Engineering--Installation Guidelines (Mar. 20, 1987), provide detailed guidelines for RED/BLACK isolation and TEMPEST control.
One method of meeting stringent electromagnetic (RED/BLACK) isolation and TEMPEST control requirements is to separate classified and unclassified equipment with a barrier that functions as a Faraday shield to confine electromagnetic radiation. Preferably, the barrier is totally impervious to electromagnetic radiation. However, since data transmission between classified and unclassified equipment is often necessary, a totally impervious barrier between the two types of equipment is generally not possible.
Another technique, which provides electromagnetic isolation and still permits data transmission across a classified/unclassified (RED/BLACK) boundary, includes the use of an optically coupled isolator, or simply optical isolator. Generally, the optical driver device of the optical isolator is an infrared emitting diode and the optical detector device of the optical isolator is a photo sensor diode. Conventionally however, optically coupled isolators are housed in high dielectric housings which are suitable for mounting on a printed circuit board rather than for mounting through an electromagnetic barrier and do not incorporate techniques for meeting TEMPEST requirements.